I'm curious how the OW sees this. She's getting your stellar example of a good guy, which should be diminished in her eyes simply because you are focusing so much of your mind and self on someone other than your wife. But I'm thinking she just loves being the object of desire of a good guy, one that maybe wants her so much he does things like have drunken, intimate conversations by the firelight. So she can stay with her loser husband or sleep around while being on this pedestal you created. Might be a way for her to avoid finding her own decent relationship instead of siphoning one from her 'best friend's husband.
As long as you are examining your own motivations with a hard, clear eye - spend some time examining the OW's reasons too. She's being a lousy friend to your wife. You think maybe she'd love it if you slipped, allowing her a way out of a marriage she doesn't want to stay in? You might lose some of the glow-y, forbidden love feelings if you start seeing some of the flaws in her behavior.